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SOME TECHNIQUES FOR MEASURING RATES

Collection of an evolved gas (Increase in gas pressure. Titration. Electrical conductivity. SOME TECHNIQUES FOR MEASURING RATES. Light absorption. Measurement of the mass of the reaction mixture. Clock techniques. Collection of an Evolved Gas (Increase In Gas Pressure).

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SOME TECHNIQUES FOR MEASURING RATES

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  1. Collection of an evolved gas (Increase in gas pressure Titration Electrical conductivity SOME TECHNIQUES FOR MEASURING RATES Light absorption Measurement of the mass of the reaction mixture Clock techniques

  2. Collection of an Evolved Gas(Increase In Gas Pressure) The gas produced in the reaction is collected either in a gas syringe(gas collecting bottle) or in agraduated vessel over water.

  3. Allgases can be collected using a gas syringe. The syringe is connected by a tube to the flask which is giving off the gas. The gas syringe has marks along its lengthwhich allow the volume of collected gas to be measured.The increase in volume can be timedand a graph of volume against time can be plotted.This shows the rate of the reaction.

  4. Collecting GasOverWater A tube from the flask which is giving off the gas is placed under water.A test tubefull of water is placed upsidedown above the end of the tube to collect the gas. As the gas bubbles into the graduated vessel it displaces the water until the graduated vesselis full. This works well for insoluble gases such as hydrogen, or gases that do not dissolve easily in water, such as oxygen and carbon dioxide. Ammonia and chlorine are readily soluble in water and are not collected this way.

  5. An alternative technique is to carry out the reaction in a vessel of fixed volume and monitor the increase in the gas pressure. These techniques would be suitable for measuring the rate of reaction between a moderately reactive metal (such as zinc) and an acid such as HCl) Zn(s) + 2H+(aq) Zn2+(aq) + H2(g)

  6. Measurement of the Mass of the Reaction mixture The total mass of the reaction is only vary if a gas is evolved. To be effective the gas being evolved should have a high molar mass (i.e. not hydrogen) so that there will be a significant change in mass. This technique would be suitable for measuring the rate of reaction between a metal carbonate and an acid by measuring the rate of mass loss resulting from the evolution of carbon dioxide. CaCO3(s) + 2H+(aq) Ca2+(aq) + H2O(l) + CO2(g)

  7. Electrical Conductivity The presence of ions allows a solution to conduct electricity, so if there is a significant change in the concentrations of ions (hydrogen and hydroxide ions have a high conductivity) during the reaction, rate can be found from the change in conductivity. Conductivity is an index of ease of flow of electric current and the reciprocal of resistivity. Therefore conductivity can be found by measuring the AC resistance between two electrodes. The technique can be suitable for the following reaction which produces ions from non-ionic reactants: PCl3(aq) + 3H2O(l) H2PO3-(aq) + 4H+(aq) + 3Cl-(aq)

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